The present invention relates to a control apparatus for electric rolling stock driven by an induction motor which is powered through an inverter of a variable voltage variable frequency.
In recent years, an inverter of variable voltage/variable frequency (known as VVVF inverter for short) which is designed for converting a DC power to an AC power of variable voltage/variable frequency to drive an induction motor is extensively employed in the electric rolling stock such as electric railcars. In this conjunction, an output torque T of the induction motor is given by EQU T.alpha.(V/F.sub.INV .sup.2 .multidot.F.sub.S ( 1)
where
V represents a motor voltage, PA1 F.sub.INV represents an inverter frequency, and PA1 F.sub.S represents a slip frequency.
The above expression (1) means that the output torque of the induction motor can be controlled by regulating the voltage/frequency ratio V/F.sub.INV or the slip frequency F.sub.S.
In general, as a method of controlling the electric rolling stock, constant-torque control methods are adopted. According to one of the most popular constant torque control methods, the induction motor is driven over a plurality of divided regions, i.e. a region (constant torque region) in which a constant torque is derived by maintaining the ratio V/F.sub.INV and the slip frequency F.sub.S to be constant at respective predetermined values, a region (constant output region) in which after the motor voltage V has attained a predetermined maximum value, the torque T is controlled in inverse proportion to the inverter frequency F.sub.INV while maintaining constant the ratio F.sub.S /F.sub.INV (i.e. motor current I), and a region (referred to as the characteristic region) where the torque T is controlled in inverse proportion to a square of the inverter frequency F.sub.INV while maintaining constant the voltage V at the predetermined maximum value and the slip frequency F.sub.S at a constant value.
As an approach for operating the inverter in conformance with the characteristics of a given induction motor, there is proposed in JP-A-57-166881 an apparatus for varying the very value of the constant torque by changing the value of V/F.sub.INV in the constant torque region.
Needless to say, the performance of the electric rolling stock is determined in dependence on the types of trains and the operations for which the rolling stock is intended. By way of example, for a long-distance transit service and an express train service, an electric rolling stock which can exhibit an improved high-speed performance at the expense of acceleration performance should be selected, while for a short-distance commuter train service, such an electric rolling stock should be selected which is excellent in respect to the acceleration behavior rather than the high-speed performance. For a medium-distance express train service, an electric rolling stock having medium performances may be selected.
Such being the circumstances, there will arise no problems so far as the electric rolling stock having the performances such as mentioned above are used for the applications and purposes to which they are intrinsically intended. At the present state, however, electric rolling stock having fixed performances actually employed in a variety of train operations and services. By way of example, a short-distance commuter rolling stock is used for the intermediate-distance high-speed or express train service, while an electric rolling stock designed for an intermediate-distance high-speed or express service is employed as a short-distance commuter train.
When an electric rolling stock is used for the train operations and the services or transit to which the intrinsic performance of the electric rolling stock is not suited, the latter will necessarily be operated in the region or condition for which it can exhibit only poor performance. Secondly, it is noted that the induction motor is often imposed with heavy loads because the electric rolling stock is frequently operated in the region for which the performance of the motor is poor.
It is now assumed, for example, that a short-distance commuter train type electric rolling stock is used for an intermediate distance train service. In this conjunction, it is noted that the induction motor is generally imposed with a maximum permissible rotation speed. Consequently, the above assumption means that electric rolling stock having a high gear ratio is operated in a high-speed range with an increased frequency. To say in another way, the short-distance commuter train type electric rolling stock is operated over an extended time at a speed in the vicinity of the maximum rotation number of the induction motor. In that case, the use life of the induction motor will apparently be shortened. Besides, due to occurrence of significant vibrations during the operation, individual parts of the driving unit tend to become loose. Further, the poor acceleration performance in the high-speed range is accompanied with considerably bad economy.
On the other hand, when an electric rolling stock designed for an intermediate-distance train service is used in a short-distance train, acceleration mode in the start or initial running phase is effectuated frequently, which will eventually incur an overheat failure of the induction motor. This problem may be solved by increasing the number of the electric rolling stocks with the respective induction motors which are employed in a train, which is however very expensive.